The invention provides a marker for determining sensitivity of a subject having cancer to an anti-cancer agent in a biological sample derived from the subject, said marker being one or more selected from cysteine-glutathione and guanosine, and said anti-cancer agent being selected from oxaliplatin, fluorouracil, and a salt thereof. The invention also provides a method for determining sensitivity of a subject having cancer to an anti-cancer agent selected from oxaliplatin, fluorouracil, and a salt thereof, comprising measuring the level of a substances selected from cysteine-glutathione and guanosine in a specimen derived from the subject.
